Hormones function as extracellular stimuli that activate a cascade of intracellular pathways regulating growth, differentiation, and the immune response. Our long range goal is to understand the intracellular pathways that are activated in response to prolactin. Prolactin, an anterior pituitary hormone, exerts diverse biological effects on a variety of target cells. Experiments with animals have revealed two physiological effects of prolactin; it induces the appearance of breast cancer, and it is required for development of the immune system. Prolactin appears to elicit these biological responses by altering specific gene expression. In T lymphocytes, we discovered a prolactin-induced DNA binding factor that recognizes a specific sequence present in the promoter of prolactin- stimulated genes. We determined recently that this factor contains two members of the emerging family of signal transducers and activators of transcription (STATs). In T cells prolactin activates both STAT5, originally known as mammary gland factor, and STAT1, originally identified as an interferon-induced transcription factor. Knowledge of the molecular mechanism of STAT activation by prolactin is critical to our understanding of the physiological effects stimulated by prolactin. We plan an integrated approach to study the prolactin signal transduction pathway that leads from the cell surface receptor to the nucleus in order to determine the mechanism of activation of STAT5 and STAT1, and to define their role in specific gene expression. These studies should provide information necessary intercept or activate signals elicited by prolactin in breast cells and T lymphocytes.